s390: add a few more SPDX identifiers

Add the correct SPDX license to a few more files under arch/s390 and
drivers/s390 which have been missed to far.
The SPDX identifier is a legally binding shorthand, which can be used
instead of the full boiler plate text.

Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>
diff --git a/arch/s390/include/asm/segment.h b/arch/s390/include/asm/segment.h
index 8bfce34..97a0582b 100644
--- a/arch/s390/include/asm/segment.h
+++ b/arch/s390/include/asm/segment.h
@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: GPL-2.0 */
 #ifndef _ASM_SEGMENT_H
 #define _ASM_SEGMENT_H